The line that best reflects what the necklace symbolizes at the end of the story is:
"Madame Forestier, deeply moved, took her hands in her own. 'Oh, my poor Mathilde! But mine was imitation. It was worth at most five hundred francs!'"
This line reveals that the necklace, which Mathilde believed to be a valuable diamond necklace, was actually a fake. This twist emphasizes the themes of appearance versus reality and the costs of vanity and pride, illustrating how Mathilde's pursuit of a glamorous life led to her tragic downfall.